Inducing Ovulation With Recombinant Equine Luteinizing Hormone

Manipulating the estrous cycle in mares is a big part of many breeding operations, and medications play various roles in the process. There are several options for inducing ovulation, including human chorionic gonadotropin (hCG), gonadotropin-releasing hormone (GnRH), and recombinant equine luteinizing hormone (reLH).

Kory Niswender, DVM, MS, Dipl. ACT, of Reata Equine Hospital in Weatherford, Texas, discussed a study on reLH at the 52nd annual American Association of Equine Practitioners Convention, held Dec. 2-6, 2006, in San Antonio, Texas. The goal of the study was to determine an effective dose of reLH and compare its effectiveness to the currently used hCG.

Theoretically hCG and reLH should induce similar responses in the mare, since hCG mimics the activity of luteinizing hormone and reLH is an engineered form of luteinizing hormone. (Luteinizing hormone hastens the maturation and induces ovulation of the dominant ovarian follicle in a mare.)

There are some concerns with hCG and GnRH use, Niswender said: "Mares become refractory to hCG over time (it is recognized as a foreign molecule and they develop antibodies to it, so it becomes less effective after multiple uses). And there have been downregulation problems in the past when GnRH implants were used (some mares had a delayed return to estrus if they did not get pregnant on that cycle)."

In contrast, he reported that no antibodies were found to reLH in the initial safety study of this project, and one mare ovulated within 48 hours on each of six treated cycles, so the hormone's effectiveness should not decrease with multiple uses.

Eighty-five light-breed horses from three to 13 years of age were used for this study. They were randomly assigned to receive saline solution (control group); 300, 600, 750, or 900 micrograms of reLH; or 2,500 IU hCG intravenously. They were treated once they had follicles of 35-39 mm in diameter and uterine edema (fluid swelling, a reliable indicator of heat in the normal mare). Mares were frequently monitored with ultrasound to determine when they ovulated following treatment.

The ideal treatment dose of reLH was found to be 750 micrograms; at this dosage, nine of 10 treated mares (90%) ovulated within 48 hours. The 900-microgram dose was nearly as effective with 16 of 20 mares (80%) ovulating in 48 hours. Of the seven mares treated with hCG, six (86%) ovulated within 48 hours.

"The 750- and 900-microgram doses worked equally well at inducing ovulations in 48 hours, but the 750-microgram dose offers similar results with less hormone," Niswender summarized. "We did visualize good-sized corpora lutea and progesterone levels were high, so reLH treatment didn't give rise to any problems with corpus luteum formation or function. (The corpus luteum is what remains of a follicle after an egg has ovulated; it produces the hormone progesterone late in the estrous cycle and into pregnancy if the egg is fertilized.) We do need more mares to confirm these results."

During the question and answer session, Niswender noted that he had not seen any deleterious effects of reLH on pregnancy rates, and that the medication can be obtained from Bioniche Animal Health for approximately $14 per dose.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse'sAAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

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